1. Field of the Invention
The present invention relates to heating towers that provide heated water to satisfy a predetermined flow demand and more particularly, to a system that selects primary and secondary heating towers from a plurality of heating towers to satisfy the flow demand.
2. Background of the Prior Art
Control systems that start-up, operation and shut-down heating towers are commonly used. The control technologies use electrical relays with discrete components, programable logic controllers (PLC) and computers. The systems developed to control the heating towers include the use of dedicated primary and secondary heating towers to satisfy flow demands at a predetermined temperature. Generally, the dedicated secondary heating tower starts and stops correspondingly with flow demand or based upon other predetermined parameters. Dedicated primary heating towers are not used as secondary heating towers even if the primary tower would be less expensive to operate as a secondary tower. Further, irrespective of the wear and time of operation on the dedicated primary towers, a dedicated secondary tower is not used as a replacement for the primary towers.
The problem with the prior art heating tower control systems is that the systems do not have the flexibility to select any available tower as either a primary tower or a secondary tower. Not having this flexibility increases the quantity of heating fuel ultimately used to heat liquids (water) required to satisfy flow demand rates. The increased fuel use results from less available turndown ratio for multiple heating towers (turndown being the additive capacity of all heating towers combined divided by the minimum fire capacity-low firing rate of the smallest heating tower). Also, the wear and fatigue that develops upon a primary heating tower in continuous use cannot but distributed to less used towers, but instead increases until the respective dedicated primary tower unexpectedly fails.
A need exists for a control system for heating towers that allows any tower of a plurality of heating towers to be selected as either a primary or secondary heating tower. The control system must be able to select single or multiple heating towers as being primary or secondary. Further, the selection of any heating tower for use must be based upon parameters that quantify heating tower wear. Also, the control system must be capable of starting and stopping secondary heating towers based upon flow demand thereby reducing cost for heating the liquid.
It is an object of the present invention to overcome many of the disadvantages associated with control systems for heating towers.
A principle object of the present invention is to provide a system that independently controls heating towers irrespective of the tower being a primary or secondary heating source. A feature of the system is that no heating tower is permanently designated as primary or secondary. An advantage of the system is that the turndown ratio, being defined as the maximum high fire (the additive capacity of all heating towers combined)/the minimum fire capacity (low firing rate of the smallest heating tower), is increased thereby reducing fuel consumption when heating liquids (water) to satisfy flow demand.
Another object of the present invention is to alternate the selection of primary and secondary heating towers amongst all of the heating towers. A feature of the system is to shut down secondary heating towers when flow demand is such that only the primary heating tower need operate to satisfy the flow demand. An advantage of the system is that fuel consumption is reduced. Another advantage of the system is that operating wear is more evenly distributed amongst all the heating towers.
Still another object of the present invention is to select multiple primary and multiple secondary heating towers meet flow demand. A feature of the system selects the heating tower with the smallest heating capacity when satisfying flow demand. An advantage of the system is that operating costs are reduced when satisfying demand.
Yet another object of the present invention is to select primary and secondary heating towers with the least wear. A feature of the system is the collection of fatigue or wear parameters for each heating tower including but not limited to inoperable heating towers, running time, number of stops and starts, number of failures (unexpected shutdown), and the time of day with corresponding low and high flow demand. An advantage of the system is that the useful life of all the heating towers is increased.
Another object of the present invention is to maintain the operation of a selected primary tower while allowing multiple secondary to stop and start depending upon flow demand. A feature of the system is utilize a secondary heating tower that minimally meets flow demand which exceeds the capacity of the primary heating tower. An advantage of the system is that use of heating fuel to satisfy the flow demand is kept to a minimum.
Briefly, the invention provides a control system for multiple heating towers comprising means for selecting at least one of a plurality of heating towers as a primary heating tower; means for starting said selected primary heating tower; means for operating said selected primary heating tower; means for operating at least one of a plurality of heating towers as a secondary heating tower; means for starting said selected secondary heating tower when predetermined demand parameters are required of said selected primary heating tower; and means for operating said selected secondary heating tower.
Further, the invention provides a method for heating liquids and distributing operating wear amongst a plurality of heating towers, said method comprising the steps of determining operable heating towers form a plurality of heating towers; determining initial flow demand; determining the temperature that the initial flow demand is to be heated; selecting at least one primary heating tower from said operable heating towers, said selected primary heating tower satisfying initial flow demand at said determined temperature; starting said selected primary heating tower; operating said selected primary heating tower; determining when flow demand has increased beyond the heating capacity of said primary heating tower; selecting at least one secondary heating tower from said operable heating towers, said selected secondary heating tower satisfying increased flow demand at said determined temperature; starting said selected secondary heating tower; and operating said selected secondary heating tower.